I use the Garmin 405 which tracks "everything" but I'd say at least keep track of your time and get as close as you can with the distance. Then, input the two values into a pace calculator to determine your pace, too. It's important to get a feel for your average pace. For example, I'm almost at a point where I can tell how fast I'm running just by how I feel because I've become familiar with the data, so if I run for X minutes at X pace, I know I've run X miles. Helpful?

We have a free training log for you to help you keep track of all your data: http://www.activetrainer.com/endurance/product/at-basic

The Garmin is a pretty cool training tool, but you don't need to go all out and buy the most expensive model. In fact, the 305 can be found online for ~$100